Actin and myosin are involved in a host of contractile events in nonmuscle cells including cytokinesis, phagocytosis and cell locomotion. The regulation of the interaction of these two proteins is critical to normal cell function. There are at least two possible regulatory systems involved in this. One of these is the myosin light chain kinase-dependent phosphorylation of myosin. This kinase is regulated by calcium and calmodulin. Myosin must be phosphorylated in order to be activated. The other system involves caldesmon, an actin and calmodulin binding protein which can inhibit myosin's activity in vitro. The inhibition can be reversed by calmodulin binding to caldesmon. Caldesmon is a substrate for various kinases and has been shown to be phosphorylated during smooth muscle contraction. It is also phosphorylated when human platelets are treated with prostaglandins at sites phosphorylated in vitro by cAMP-dependent protein kinase. We have examined the sites phosphorylated on caldesmon by this kinase and find that they are in the amino-terminal region of the molecule. We have also started to study the role of contractile proteins in the agonist-induced contraction of human microtrabecular cells. These cells have many characteristics that make them very suitable for these, types of studies. They contain a large amount of myosin, have a well spread phenotype in culture, contract to about one-half of their original surface area upon addition of agonist in a time frame of about 20 minutes and can be easily grown in culture.